A 485cm sediment core from a lake uno cially called JR01, Boothia Peninsula, Nunavut, Canada, yielded a high-resolution diatom record documenting environmental change in the mid-arctic. Five radiocarbon dates provided the chronology. Changes in diatom composition and sediment character- istics indicated distinct shifts in the Holocene climate. A more diverse and productive diatom ora implies warmer temperatures in the middle Holocene. A subsequent complete shift in diatom composition to a predominantly Fragilaria sensu lato ora and a reduction in diversity and production suggests cooler cli- mates in this region after 4600cal. BP. Smaller-scale climatic uctuations, such as the ‘Little Ice Age’ (LIA, 600–150cal. BP) and the ‘MediaevalWarm Period’ (MWP, 1150–600cal. BP), caused shifts in the diatom ora and production. Subtle shifts in oristic diversity within the LIA may re ect climatic variability at a century scale. A gradual shift to a more diverse and productive ora in the last 150 years suggests a response to the recent warming trend.
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